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The drug works by improving insulin sensitivity, promoting the uptake of glucose into tissues and lowering sugar levels in the bloodstream. By increasing how effectively the existing glucose is used, metformin reduces the amount of glucose the liver produces and the intestines absorb.

Alcohol also affects blood sugars significantly. Alcohol digestion puts stress on the liver, an organ dedicated to the removal of poisons from the body. When the liver is forced to process high amounts of alcohol, it becomes overworked and releases less glucose.

Long-term alcohol use can also make cells less sensitive to insulin. This means that less glucose is absorbed from the blood and levels in the bloodstream increase.

Over time, alcohol consumption damages the liver, especially when it is consumed in excess. It reduces the liver's ability to produce and regulate glucose. Conditions like alcoholic hepatitis and cirrhosis of the liver can occur with chronic alcohol use, greatly decreasing liver health and impairing blood glucose control.

Most alcoholic drinks also contain high amounts of sugar. These can contribute to conditions that influence blood sugar control, such as being overweight. Many alcoholic drinks are also carbonated, meaning that they raise blood sugar levels even more quickly.

Although an occasional drink may not cause any damage, the potential side effects are likely to outweigh the benefits. Anyone taking metformin should talk to their doctor about using alcohol while taking the medication.

Side effects

Gastrointestinal complications are the most common side effects of metformin. An estimated 1 in 10 people taking metformin will experience symptoms.

Many of the side effects of metformin are the same as alcohol, so mixing the two can intensify symptoms. The extent to which alcohol influences metformin side effects depends on how much alcohol is consumed and individual health factors.

In general, the more alcohol consumed, and the quicker it is ingested, the greater the influence it has.

Common metformin side effects made worse by alcohol use include the following:

Alcohol puts stress on the liver, so the liver is unable to produce enough glucose.
This can cause mild or serious side effects.

Many of the lesser symptoms of metformin can be relieved by taking the medication with food. Drinking enough fluids can also reduce the symptoms. Once the body adjusts to the medication, many side effects tend to resolve themselves.

Less than 1 in 10,000 on metformin will experience flushing of the face, or redness from increased blood flow. This is a symptom shared with alcohol use.

Complications

While the individual risks vary and depend on additional health factors, people with diabetes who consume alcohol while on metformin can experience life-threatening complications.

Lactic acidosis

Energy is mainly produced in the muscles using oxygen-dependent processes. During strenuous or prolonged activity, oxygen demand can outweigh supply. This causes cells to turn to anaerobic, or oxygen-lacking, processes.

Anaerobic glucose breakdown produces lactic acid, which is further broken down into lactate. Lactate is broken down into glucose by the liver.

Lactate levels can rise during extended exercise or strenuous activities as oxygen is required to help clear it. When lactate does not clear from the bloodstream quickly enough, it can build up, increasing blood and muscle acidity. When lactate levels are too high, lactic acidosis occurs.

Metformin slows the rate of lactate uptake by the liver, as does alcohol. The risk of developing lactic acidosis while on metformin alone is quite rare, at around 0.0001 percent. When taken alongside alcohol, the risk increases significantly.

Signs of lactic acidosis can be subtle and nonspecific at first, such as gut pain and sleepiness. They can also be easily mistaken for signs of alcohol consumption.

Severe cases have intense symptoms that are quick to appear, however. Lactic acidosis can be life-threatening. If symptoms occur, people should seek medical attention immediately.

Symptoms in mild hypoglycemic cases, such as headaches, tiredness, and hunger, are usually too vague to be a warning sign. Symptoms of low blood sugar are also easy mistaken with signs of alcohol consumption, meaning low blood sugars can be missed while drinking.

In severe cases, these symptoms are more acute and can become life-threatening. If symptoms are intense or worrisome, people should seek medical attention immediately.

The warning signs of low blood sugar include:

There are a number of indicators of low blood sugar that people with diabetes should watch out for.

If low blood sugar symptoms occur, people with diabetes should check their blood glucose levels. Lowered blood sugars can often be corrected at home using glucose supplements or consuming 15 grams of simple sugars, such as honey or fruit juice.

If blood sugars have not been restored after 15 minutes, additional doses should be consumed until normal levels are reached.

Drinking alcohol before bedtime can lead to blood sugar dips during the night. People with diabetes should eat a complex carbohydrate alongside or after alcohol intake to avoid this problem.

Vitamin B12 deficiencies

Some studies suggest metformin users have a 0.0001 percent chance of developing a vitamin B12 deficiency. However, more recent research suggests that the risk may be much higher, with 10 to 30 percent of people with long-term type 2 diabetes, who are on metformin, experience reduced circulating B12 levels.

Vitamin B12 is an essential nutrient, key to cardiovascular and neurological health. Vitamin B12 is also a vital component of healthy red blood cells.

While the symptoms may be subtle and slow to progress, significant B12 deficiencies can pose serious health risks. If a B12 deficiency is suspected, people should seek medical advice.

The warning signs and complications of vitamin B12 deficiencies include:

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